Better technology, less carbon dioxide

As one of the Nordic region’s leading energy companies, Fortum has prioritised carbon dioxide-free capacity in its generating portfolio for some time. It is now looking in more detail at how carbon capture and sequestration technology can be developed for use at the company’s existing and future fossil-fuelled plants.

In a world where people are becoming increasingly concerned about the rate at which we are depleting non-renewable energy resources, coal remains in good supply and continues to be the most widely used fuel for power and heat, generating around 40% of electricity worldwide.

Burning coal using even advanced combustion technology, by today’s standards, however, produces carbon dioxide, a major greenhouse gas. A lot of work is under way to address this challenge, and scenario studies indicate that carbon capture and sequestration would be the most effective technology for reducing CO2 emissions in the power generation sector over the next couple of decades.

The most widespread carbon capture concept at the moment is based on removing carbon dioxide from a plant’s flue gases. The gas thus captured then needs to be transported off-site, either in a pipeline or by ship, to secure storage.

Storage options include oil fields or deep saline aquifers. Injecting gas into old oil fields can enhance extraction, and carbon dioxide is already being transported for this purpose along pipelines in Texas. Carbon dioxide from the Sleipner field in the Norwegian sector of the North Sea has also been injected into the Utsira aquifer formation, and has been successfully contained there with no leakages for around 10 years.

If carbon capture is widely applied in the future, however, the amount of storage capacity called for will be quite massive.

Technology in flux

The EU is actively driving technology development in this area, and is keen for industry to develop an ambitious, large-scale carbon capture and sequestration demonstration programme between 2010 and 2020.

A project with Sargas is under way at Fortum Värme’s CHP plant in Stockholm to capture CO2 using potassium carbonate.

Fortum is interested in these technologies, not only because of their potential in mitigating climate change, but also because its generating portfolio could contain more coal- and/or gas-fired capacity in the future as the company grows. Given that carbon capture technologies are still in flux, Fortum is keeping an open mind on all of the three main options currently being developed: post-combustion capture, oxyfuel combustion, and pre-combustion capture.

In the first of these, chemicals such as amines, potassium carbonate, and chilled ammonia are employed in scrubbers installed either as retrofits or in greenfield projects.

In the case of oxyfuel combustion, combustion air in the boiler is replaced by a mixture of oxygen and recirculated carbon dioxide. This keeps flue gases free of nitrogen oxides and enables CO2 to be removed relatively easily. This approach is being developed mainly for greenfield applications, but some retrofit potential exists.

Carbon dioxide can also be removed from an integrated coal gasification combined cycle (IGCC) plant before the gas turbine, firing the latter mainly on hydrogen. This makes IGCC plants ‘capture-ready’, in the sense that carbon capture can be implemented at a later point in time, as and when needed, without massive modifications to the original plant.

The challenges

There are some significant challenges in bringing the technology needed for carbon capture and sequestration at large-scale power plants to a commercially viable level, however. These include resolving the problem caused by the fact that today’s absorption methods and oxyfuel combustion technologies consume a significant proportion of the power output of a plant and result in a considerable reduction in overall efficiency.

There are also unresolved challenges in how to control a boiler operating in oxyfuel combustion mode, as well as investment cost and reliability issues with IGCC technology. The costs and energy consumption associated with producing the oxygen needed for both gasification and oxyfuel combustion also still need to be addressed. Neither has any consensus been reached on realistic purity requirements for captured carbon dioxide.

Addressing the practicalities

Fortum has development work under way in all the main areas of carbon capture. A carbon capture test facility is being developed with Sargas at the Värtan plant in Stockholm in Sweden, which features a pressurised bubbling fluidised bed boiler, for example, based on using potassium carbonate to absorb CO2, as this reagent offers a number of advantages over amines.

Fortum is also cooperating with boiler manufacturers, such as Hitachi, and research institutes, such as VTT Technical Research Centre of Finland, to investigate the feasibility of oxyfuel combustion at Fortum plants. This has included experimental work, computational fluid dynamics (CFD) simulations, and preliminary layout studies.

Combustion behaviour in oxyfuel conditions compared to current air combustion systems has been a particular focus of attention, with analysis of boiler heat radiation, steam performance, and NOx emissions. Experimental combustion tests have been used to validate simulation models.

CFD modelling carried out on one of Fortum’s boilers has shown that it could be operated safely in oxyfuel mode, with good flame stability. The colours indicate flame temperature in centigrade.Click here to enlarge picture